(1) Field of the Invention
The present invention relates to an optical access network system and, more particularly, to an optical access network system where a passive optical network (PON) connects a central office side apparatus and a plurality of subscriber connection apparatuses connected to subscriber terminals and each subscriber connection apparatus transmits data during a transmission grant period assigned from the central office side apparatus.
(2) Description of Related Art
Recently, higher speed access networks have been developed, required by the proliferation of Internet and an expanding demand for service of access to a huge volume of data such as video streams to be delivered. As measures for providing a higher speed access network, so-called Fiber to the Home (FTTH) schemes in which optical fibers are used as subscriber lines are becoming a mainstream. As one FTTH scheme, attention focuses on a PON system where an optical fiber is shared by a plurality of subscribers. The PON is also called a Passive Double Star (PDS) or Point to Multipoint (P-MP).
In the PON system, a plurality of branch optical fibers which are connected, respectively, to subscriber connection apparatuses, Optical Network Units (ONUs; slave stations), are joined into a single optical fiber by a star coupler (optical splitter) and the optical fiber between the star coupler and a central office side apparatus, Optical Line Terminal (OLT; master station) is shared by a plurality of subscribers. Thus, the PON system enables a great reduction in the optical fiber infrastructure cost. The PON system is classified into, for example, a Broadband PON (B-PON) for transmitting information in ATM cells, a Giga-bit PON (G-PON) enabling high-speed data transfer in the order of gigabits per second, and a Giga-Ethernet PON (GE-PON) appropriate for Ethernet service.
In the PON system, data transmission (downstream communication) from the master station to slave stations is performed in a multicast communication manner and data packets or frames transmitted from the master station are distributed by the star coupler to the plurality of branch optical fibers. Each slave station interprets the destination address of each received packet and selectively forwards only packets addressed thereto to its destination subscriber device. On the other hand, data transmission (upstream communication) from each slave station to the master station is performed in time division multiplexing. Data packets or frames transmitted over the branch optical fibers from each slave station are multiplexed onto a single optical fiber by an optical coupler. To avoid collision of upstream data packets on the optical fiber in the PON system, upstream data transmission is controlled from the master station by assigning a packet transmission grant period to each slave station.
Branch optical fiber sections in the PON are unequal in length and upstream signal propagation time from each slave station to the master station varies. Therefore, it is needed to know in advance the upstream signal propagation time from each slave station in order to assign an appropriate packet transmission period to each slave station and to prevent collision of packets passed through the branch optical fibers and multiplexed on the shared optical fiber. When a new slave station has been connected to the PON system, a special communication takes place to realize the functions of detecting the slave station by the master station, measuring the propagation time between the slave station and the master station, and registering in the master station the identifier of the slave station and the propagation time. This series of communication is called a ranging process. The signal propagation time measured for each slave station is stored into a management table on the master station.
The master station assigns a transmission grant period to each slave station in response to a data transmission request from the slave station. For example, in the B-PON system where upstream communication is performed in units of a cell having a fixed length of 56 bytes, i.e., a 53-byte ATM cell plus a 3-byte overhead, the master station periodically inserts a transmission grant signal in downstream traffic, thereby to notify each slave station of time slot numbers. In the G-PON that is capable of carrying variable-length packets, a transmission grant period is specified with a set of a transmission start time and a transmission end time. In the GE-PON, a transmission grant period is specified with a transmission start time and the number of bytes to be occupied. Approaches for management of upstream packet transmission timing in the PON are described in, for example, Yokotani, Mukai, Oshima, “Queuing Models for Media Access Control on PON,” The Institute of Electronics, Information and Communication Engineers (IEICE), Shingakugiho CS2003-109, pp. 7-10, November 2003 (Non-patent document 1) and Yokotani, Murakami, Mukai, Oshima, “Media Access Control in PON System for Optical Access Network,” The IEICE, Shingakugiho IN2003-25, pp. 7-12, June 2003 (Non-patent document 2).
However, a problem associated with the PON system using an optical fiber and a plurality of branch optical fibers as its transmission path is deterioration of signal waveforms due to optical dispersion along the optical fibers.
FIG. 2A shows a logical sequence of upstream packets that are time division multiplexed on the optical fiber. P(A) to P(N) denote packets transmitted from slave stations A to N through branch optical fibers with different lengths. For example, in the case of intensity modulation light, these packets are physically propagated on the optical fiber as optical waveforms S(A) to S(N) with light intensity levels changed, as shown in FIG. 2B.
An optical signal propagating through an optical fiber comprises, strictly speaking, a plurality of wavelength components (spectrum spread) and its group velocity has wavelength dependency. This wavelength dependency is generally called dispersion (group velocity dispersion). A dispersion characteristic has a great effect on signal waveforms passing through the optical fiber. This is because, in the optical fiber with a wavelength-dependent group velocity, a wavelength component traveling slowly and a wavelength component traveling fast appear, which results in spread waveforms. If the value of dispersion is zero or negligibly small, no waveform distortions caused by dispersion occur and rectangular waveforms are kept, as shown in FIG. 2B. However, if the value of dispersion is not negligible, waveform distortions occur in optical signals. The waveform distortion becomes larger in a signal light passed through a longer fiber, because dispersion is proportional to fiber length.
FIG. 2C shows emphasized waveform distortions caused by dispersion. No waveform distortion occurs in a transmission signal S(A) from a slave station A at a short distance from the master station, whereas a waveform distortion occurs in a transmission signal S(B) from a slave station B at a long distance from the master station. A greater waveform distortion occurs in a transmit signal S(N) from a slave station C at the longest distance from the master station. If the value of dispersion is great and the distance of optical signal propagation is extremely long, there is a possibility that the waveform is distorted to a degree that received data becomes indiscriminative at the master station.
To describe quantitatively, in the case of Single Mode Fibers (SMF) which are the most popular optical fibers, the value of dispersion per distance is approximately 17 ps/nm per kilometer and the value of dispersion for a 20 km long optical fiber reaches 340 ps/nm. If a data transmission rate is 10 Gbps, spectrum spreading in the order of at least 5 GHz occurs. This is equivalent to waveform spreading of about 0.04 nm.
A 10 GHz optical signal has a pulse width of about 100 ps, but a pulse spread of about 14 ps occurs in a 20 km long SMF from an estimation of 340 ps/nm×0.04 nm=14 ps. This value is calculated when a transmission signal is optically modulated with a relatively expensive external modulator at a slave station. The pulse width spreading will be greater than the above value, if an inexpensive optical transmitter that directly modulates a light output from a semiconductor laser is used. Consequently, when upstream packets arrive at the master station, there is a possibility that adjacent pulse waveforms overlap together and it becomes hard for the master station to correctly discriminate a 100 ps wide pulse from a pulse sequence received.
However, the above signal waveform spreading caused by dispersion has an almost negligible effect on current PON systems (B-PON, GE-PON, and G-PON), since the existing PON systems use, as an upstream signal wavelength, a 1.3 μm band in which dispersion along a fiber is approximately zero and the transmission rate applied in the PON systems is in the order of 155 Mbps to 2 Gbps.
If an inexpensive optical device with a broad spectral width such as, e.g., a Fabri-Perot Semiconductor Laser (FP-LD) is applied in an optical transmitter at a slave station for the purpose of reducing equipment cost, signal waveform distortion caused by dispersion along an optical transmission path will become non-negligible. In future, if the transmission rate will rise to, e.g., 10 Gbps or higher to further speed up an optical access network, or if a signal light in a 1.5 μm band in which dispersion is inevitable will be used as a wavelength for upstream communication, the same problem as described above will occur.
The influence of signal waveform distortion caused by dispersion can be avoided by applying a dispersion compensator, e.g., a dispersion-compensating fiber having a value of dispersion opposite to that along the optical transmission path. In the PON system, dispersion compensation that varies for each slave station is required because of different lengths of branch optical fibers from the slave stations to the star coupler.
Therefore, for example, in the case where dispersion compensation is performed at the master station, it is needed to adjust the amount of compensation for each received packet, that is, for each of the optical signals S(A) to S(N) shown in FIG. 2C by applying a dispersion compensator with a variable amount of compensation (dispersion).
In this case, if the master station tries to receive information from all packets arrived thereto varying its signal property at a high speed in the order of 100 ns, the amount of compensation (dispersion) of the dispersion compensator must be switched at a higher speed in the order of 10 ns and the master station must receive the next transmission packet after rapidly stabilizing the characteristics of the dispersion compensator. However, an optical device for variable dispersion compensation enabling such a high-speed control does not achieve on a practical level as it is now. Meanwhile, an individual dispersion compensator may be installed at each slave station, for example, at the slave station end of each branch optical fiber. In this case, dispersion compensators as many as the number of slave stations are required and, as a result, the system cost increases.